1. Field of the Invention
This invention relates to improvements in integrated circuits and integrated circuit techniques, and more particularly to improvements in methods and circuits for providing an accurate average voltage sensing for power transistors, or the like.
2. Relevant Background
Power transistors, particularly MOS power transistors used in integrated circuits, generally have source and drain active regions formed in a semiconductor substrate. Typically, for example, the source region may be an implanted or diffused region, usually having a generally rectangular shape when viewed from above, and a "U" cross sectional shape, when viewed from the side. The same is true for the drain region. The source and drain regions are separated by a channel region, with the current flow controlled therein by a potential applied to a gate located thereover, in a known manner.
Most power transistors operate to carry a high current, and in a linear region for MOS devices or in a saturation for bipolar devices. The voltage drop along the structures of big power devices causes a large difference in drain-source (or collector/emitter) voltage. When a sense transistor is used to sense a portion of the current of a power transistor, its drain-source voltage must be equal to the average drain-source voltage of the power device so an accurate current ratio can be achieved. This average point is hard to pinpoint exactly, because of many factors, such as the direction of current flow, variations in the width of metal interconnects or parts, locations of contact pads, variations in the amount of current conducted by each point on the metal interconnects, and son on.
What is needed, therefore is a circuit for enabling voltage, in general, and average voltage, in particular, to be accurately sensed across a power transistor and method for accomplishing same.